1. Field of the Invention
The present invention relates to a control channel transmission/reception method and apparatus in a wireless communication system and, in particular, to a method and apparatus for transmitting/receiving downlink control channel for increasing the coverage of the control channel of a wireless communication system.
2. Description of the Related Art
Mobile communication systems are currently evolving from basic communication devices into high-speed, high-quality wireless packet data communication systems that provide data services and multimedia services beyond the early voice-oriented services. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), both defined by 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined by 3rd Generation Partnership Project-2 (3GPP2), and Institute of Electrical and Electronics Engineers (IEEE) 802.16, have been developed to support the high-speed, high-quality wireless packet data services.
The existing 3rd generation wireless packet data communication system, such as HSDPA, HSUPA and HRPD, uses such technologies as an Adaptive Modulation and Coding (AMC) method and a channel-sensitive scheduling method in order to improve transmission efficiency. With the use of the AMC method, a transmitter can adjust the amount of transmission data according to the channel state. For example, when the channel state is not good, the transmitter reduces the amount of transmission data to match a reception error probability to a desired level, and when the channel state is good, the transmitter increases the amount of transmission data to efficiently transmit a large volume of information while matching the reception error probability to the desired level. Using the channel-sensitive scheduling resource management method, the transmitter, since it selectively services a user having a superior channel state among several users, can increase in the system capacity, as compared with a transmitter that allocates a channel to one user and services the user with the allocated channel. Such capacity increase is commonly referred to as a multi-user diversity gain. In brief, the AMC method and the channel-sensitive scheduling method are methods for receiving partial channel state information being fed back from a receiver, and applying an appropriate modulation and coding technique at the most efficient time determined depending on the received partial channel state information.
Recently, intensive research is being conducted to replace Code Division Multiple Access (CDMA), which is the multiple access scheme used in the 2nd and 3rd generation mobile communication systems, with Orthogonal Frequency Division Multiple Access (OFDMA) in the next generation mobile communication system. 3GPP and 3GPP2 have started standardization work on evolved systems based on OFDMA.
With the implementation of the ACM and channel-sensitive scheduling schemes, the base station is capable of allocating radio resource including frequency, timing, and power in adaptation to the user-specific channel condition. The base station notifies the user of the resource allocated in adaptation to the channel condition via Physical Downlink Control Channel (PDCCH) such that the user can be aware of the radio resource allocated to itself.
The radio resource allocations can be categorized into downlink resource allocation for the transmission from the base station to the user equipment and uplink resource allocation form the transmission from the user equipment to the base station. The downlink resource allocation is performed in adaptation to the channel condition reported by the user and the information amount of the data to be transmitted to the user, it is notified of the resource allocated to a certain user for transmitting data and modulation and coding scheme to be used for the data transmission via a control channel. The user equipment can recognize whether there is the resource allocated to itself and, if so, how to receive the data on the corresponding resource, based on the control information received on the control channel. The uplink resource allocation is performed in adaptation t the channel condition reported by the user and the information amount of the data to be transmitted and notifies the user of the resource allocated to the user and the transmission scheme to be used for the data transmission on the allocated resource. The user equipment can recognize the uplink resource allocated to itself and transmission scheme to be used for data transmission.
The information transmitted on the control channel for downlink resource allocation, i.e. Downlink Control Information, includes a User Equipment Identification (UE ID), a Down Link Resource Block (DL RB), a Transport Format (TF), and a Hybrid Automatic Repeat Request (HARQ).
The UE ID is the information transmitted for identifying whether there is the signal destined to the UE itself. Typically, a Cyclic Redundancy Check (CRC) associated with a certain UE ID is inserted to the DL control information such that if the UE decodes the DL control information successfully this means that the control information is transmitted to the UE.
If the UE decodes the DL control information successfully, the UE can locate the resource blocks on which the data destined to the UE are transmitted, based on the DL RB allocation information.
TF indicates the modulation and coding scheme of the transmitted signal. If AMC is adopted, the UE should know TF for performing the modulation and coding correctly.
HARQ is the process for the receiver to notify the transmitter of whether the transmission packet is received successfully so as to, if so, transmit the next packet and, otherwise, retransmit the lost packet. The HARQ-related information is the information whether the transmission signal is initial transmission or retransmission in HARQ process and the HARQ process number such that the UE determines whether to decode the currently received packet in combination with the previously received packet or perform new decoding operation based on the HARQ-related information.
The control information carried on the control channel for the downlink resource allocation can further include the information on multiple antenna transmission information, power control, and whether the distributed transmission is applied.
The information included in the control channel for uplink resource allocation, i.e. the control information, includes the UE ID, Uplink Resource Block (UL RB) allocation information, Transport Format (TF), and Hybrid Automatic Repeat Request (HARQ) related information.
The control information included in the control channel for uplink resource allocation can further include the information on the uplink reference signal for supporting Space Domain Multiple Access, the information on whether the distributed transmission is applied, and the information whether the channel condition report is requested.
If the control information is decoded successfully, the UE recognizes the resource blocks to be used for data transmission based on the UL RB information.
The UE should know the TF to recover the transmitted signal with the demodulation and decoding schemes requested by the base station.
The HARQ-related information has been described above.
The UE performs blind decoding of available candidate group to search for the downlink control channel destined to itself among plural downlink control channels. Here, the blind decoding is to perform decoding control channel candidates to find its own control channel without information on which control channel is used for the transmission of the control channel information when the base station has transmitted the control channel among the control channel candidates defined in a group of control channel candidates.
FIG. 1 is a diagram illustrating a control channel candidate group configuration method in the conventional wireless communication system.
In FIG. 1, Control Channel Element (CCE) is a unit of logical channel constituting the control channel. CCE corresponds to a group of Resource Elements (RE) as a unit of physical channel.
How many CCEs constitute a control channel is referred to as Aggregation Level (AL). If a control channel is composed of N CCEs, AL is N; and FIG. 1 is depicted in consideration of examples of AL=1 111, AL=2 112, AL=4 113, and AL=8 114.
In the exemplary case of FIG. 1, it is assumed that the control channel uses one modulation scheme. In this case, as AL increases, a number of coded bits decreases. This means that the code rate of the control channel decreases. That is, the lower AL is, the fewer the resource for transmission of the control information is; however, the UE can receive the control information successfully under good channel condition. As AL increases, the more resources are used, but, in such as case, even the user equipment experiencing bad channel condition can successfully receive the control information. In order to improve resource usage efficiency, it is preferred to configure the control channel with low AL for the user equipment having good channel condition and with high AL for the user equipment having bad channel condition.
A number of information bits constituting the control information (DCI) can be changed depending on the type of the control information. For example, the number of bits can be increased to increase degree of freedom or decreased with the cost of reduction of degree of freedom for indicating the resource block allocation information. The number of bits of the control information can be changed according to the information to be added. In case that the control information changes in length due to different numbers of bits, the lengths of the control information can be discriminated by DCI format. Since the user equipment does not know which DCI format is used for transmitting the control information, blind decoding is applied. Although transmitted in the same channel condition, it is preferred to transmit the control channel carrying the control information in DCI format of a large number of bits with a high AL as compared to the control channel carrying the control information in DCI format of a small number of bits.
Referring to FIG. 1, 8 CCEs 100 to 107 are given. This is just for the convenience sake, but a number of CCEs can be changed at any time. The number of CCEs can be influenced by constant values such as downlink system bandwidth, a number of base station transmit antennas, and a number of downlink ACK/NACK channels for supporting uplink HARQ, and the control region information in which the values change every subframe as unit of time for scheduling.
In the exemplary case of FIG. 1, there are the control channel candidates 120 to 127 corresponding to AL=1 as denoted by reference number 111. Reference number 120 denotes the control channel configured with a CCE 0 100, and reference number 127 denotes the control channel configured with a CCE 7 107. There are control channel candidates 128 and 129 corresponding to AL=2 as denoted by reference number 112. Reference number 128 denotes the control channel configured with CCE 0 100 and CCE 1 101. There is the control channel candidate 132 corresponding to AL=4 as denoted by reference number 113. Reference number 132 denotes the control channel configured with CCE 0 101 to CCE 3 103. There is the control channel candidate 134 corresponding to AL=8 as denoted by reference number 114, and reference number 134 denotes the control channel configured with 8 CCEs of CCE 0 100 to CCE 7 107.
The method for configuring the control channel candidate per AL in FIG. 1 is based on the tree structure. The control channel candidates of AL=2 includes a set of control channel candidates corresponding to AL=1, the control channel candidates of AL=4 include a set of control channel candidates corresponding to AL=2, and the control channel candidates of AL=8 include a set of control channel candidates corresponding to AL=4.
For example, reference number 132 is PDCCH composed of 4 CCEs 0 to 3 corresponding to the control channel candidate 128 including a set of CCEs 0 and 1 and the control channel candidate 129 including a set of CCEs 2 and 3 at AL=2. According to such a tree structure, when the total number of CCEs is N_CCE, a number of control channels that can be configured at an AL is floor(N_CCE/AL). Here, floor(x) is the floor function expressing the maximum integer equal to or less than x.
FIG. 2 is a diagram illustrating a structure of a downlink subframe in a conventional OFDM-based Long Term Evolution (LTE) system.
In FIG. 2, a subframe is composed of 14 OFDM symbols 200 to 213, the three OFDM symbols 200 to 202 at the beginning of the subframe are assigned for the control channel (PDCCH), and the rest OFDM symbols 203 to 213 assigned for the data channel (Physical Downlink Data Channel (PDDCH)). The control channel region is positioned at the beginning of the subframe such that the UE can check whether the subframe carries no data destined to itself and, if so, enters micro sleep mode to reduce power consumption in the data channel region.
In case that the same amount of bit information is transmitted in the control channel and the data channel in the subframe structure as shown in FIG. 2, the energy per bit on the control channel is less than that on the data channel at the limited transmit power. This is because the resource allocated for the control channel is less than the resource allocated for the data channel in time domain such that the more frequency resource is used for control channel as compared to the data channel in the same OFDM symbol and thus the energy amount per bit decreases in the limited transmit power. After all, this means that the actual coverage of the transmitted signal of the control channel is less that of the data channel. Accordingly, although the data channel is received successfully, the user terminal may fail to receive the control channel, resulting in failure of data channel decoding. There is therefore a need of a method for increasing the coverage of the control channel to become larger than that of the data channel to overcome the aforementioned problem.